How to create loop cut through an object?
Loop cut is always made on one loop of faces.
In your example you have 2 separate loops so it cannot be cut at once with this tool.

What you can do is to make one loopcut for each face loop (inner &amp; outer):


or use Knife tool K with Z (cut through) option enabled (the Z key is not captured in the example). Hold Ctrl to make a cuts in the middle of edges or if you want to place it some were else but have it accurate press C to constrain the direction of the cut to 45 degree steps.